Some of the seat belt retractors are designed to eliminate the tension applied to the wearer's body when the wearer assumes a position in which the lumbar and dorsal portions of the wearer's back are pressed against the backrest of the seat viz. assumes a fully restrained position. These retractors usually have therein a so-called "position memory mechanism" responsive to the belt being drawn out of the retractor in excess of a predetermined length so as to be reset and permit the belt to be retracted only the predetermined length whereby on return of the wearer to the fully restrained position no tension is exerted on his or her body via the belt. One example of these retractors is clearly shown and described in U.S. patent application Ser. No. 933,068 now U.S. Pat. No. 4,172,568. The retractor of this example generally comprises a ratchet wheel disposed coaxially with a webbing take up reel and rotatable relative to the reel, an external gear positioned eccentrically to the reel and rotatable relative to the reel, a stud rigidly mounted on the external gear, an elongate slot formed in the ratchet wheel to be concentric with the wheel for receiving the stud therein, a spring extending between the external gear and the ratchet wheel to bias them in a predetermined phase, an internal gear fixedly mounted to a housing of the retractor concentric with the reel and meshed with the external gear to constitute a speed reduction mechanism, a stop swingably mounted to the housing and blocking the rotation of the ratchet wheel and means for driving the stop into and out of engagement with the ratchet wheel.